igb: Support to get 2_5G link status for appropriate media type

Since i354 2.5Gb devices are not Copper media type but SerDes, so this
patch changes the way we detect speed/duplex link info for this device.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>